1. Field of the Invention:
This invention relates to a method for producing an electrode foil for use in aluminum electrolytic capacitors.
2. Description of the Prior Art:
A conventional aluminum electrolytic capacitor has been produced as follows. First, an aluminum foil is subjected to electrolytic etching in an electrolytic solution containing chlorine ions for the purpose of enlarging the effective surface area thereof. Then, the aluminum foil thus treated is subjected to anodization (anodic oxidation) for forming an oxide film (dielectric film) on the surface thereof. The aluminum foil having a dielectric film is rolled up together with a sheet of insulating paper, and then impregnated with a driving electrolytic solution, resulting in an aluminum electrolytic capacitor.
As an electrolytic solution used for the electrolytic etching, an aqueous solution of hydrochloric acid or sodium chloride is used. Also as a current form, an alternating current, a direct current, a pulse current, or a combination thereof is used in the production on an industrial scale.
Various anodization processes for forming a dielectric film on an aluminum foil have been developed, and an aqueous solution of ammonium adipate has been widely used as an electrolytic solution for anodization processes conducted at a low voltage of 100 WV or lower. Because aluminum has a poor solubility in an aqueous solution of ammonium adipate, very little amount of aluminum is eluted into the electrolytic solution. Thus, when an electrode foil is produced using an aqueous solution of ammonium adipate as an electrolytic solution for the anodization process, the capacitance of an electrolytic capacitor using the resultant electrode foil becomes larger, as compared with the cases where other electrolytic solutions are used.
An improved anodization method using an aqueous solution of adipic acid as an electrolytic solution is disclosed in Japanese Laid-Open Patent Publication No. 61-121419. This method is used for aluminum foils previously subjected to electrolytic etching by the use of a pulse current.
The above-mentioned anodization process is usually conducted with an apparatus as shown in FIG. 1. The apparatus comprises a direct current (DC) power supply 2, an anodizing tank 3 containing an electrolytic solution 6, and a feed roller 1 made of a metal such as copper (Cu) or silver (Ag), which is disposed above the anodizing tank 3 and connected to the positive electrode of the DC power supply 2. Three electrode plates 4 are disposed in the anodizing tank 3 and connected to the negative electrode of the DC power supply 2. An insulating roller 7 is disposed in a lower section of the anodizing tank 3, and a transporting roller 9 is disposed above the anodizing tank 3.
In this conventional anodization process, aluminum foil 5 is continuously introduced downward along the feed roller 1 into the electrolytic solution 6 at a position between two adjacent electrode plates 4 in the anodizing tank 3, and transported along the insulating roller 7 to be directed upward, and conveyed through the electrolytic solution 6 between two adjacent electrode plates 4. As a result, an oxide film is formed on the aluminum foil 5, resulting in an anodized aluminum foil (electrode foil) 8. The anodized aluminum foil 8 is continuously conveyed out of the anodizing tank 3 along the transporting roller 9.
In this way, the aluminum foil 5 is continuously anodized in the electrolytic solution 6. However, an oxide film is not formed on the portion of the aluminum foil 5 which has just been introduced into the electrolytic solution 6. In other words, the aluminum foil 5 has no oxide film when it is located in area A as shown in FIG. 1. This causes a rush current flow with a density of about 3.times.10.sup.4 mA/cm.sup.2 to 2.times.10.sup.5 mA/cm.sup.2 through the aluminum foil 5 in the area A. As the aluminum foil 5 is further transported, an oxide film is rapidly grown thereon. With an increase in the thickness of the oxide film, the current density of the aluminum foil 5 in the electrolytic solution 6 is decreased.
Because of a rush current flow with such a high density, the surface irregularities of the aluminum foil which has previously been formed by etching is removed by dissolution. Thus, the effective surface area of the aluminum foil which has been enlarged by etching cannot be maintained at a satisfactory level. Also because of such a large electric current density, the resistance becomes large, thereby causing a power loss. This increases the cost of producing an electrode foil. Furthermore, the rush current with a high density generates Joule's heat, which causes a problem in that the temperature of both the electrolytic solution 6 and the surface of the aluminum foil 5 is increased.
In this conventional process, the aluminum foil 5 is continuously transported through the electrolytic solution 6 during the anodization process, and the oxide film is rapidly grown thereon. Since aluminum is eluted from the aluminum foil 5 at an extremely high rate, the eluted aluminum cannot be diffused throughout the electrolytic solution 6, thereby increasing the pH of the electrolytic solution 6 in the area near the surface of the aluminum foil 5. As a result, a large amount of aluminum hydroxide is deposited on the oxide film. With an increase in the thickness of the aluminum hydroxide film, satisfactory capacitance cannot be attained in an electrolytic capacitor using the electrode foil thus obtained.
Moreover, water in the aluminum hydroxide film reacts with aluminum in the oxide film, thereby causing deterioration in the quality of the oxide film. When an electrode foil having such a low-quality oxide film is used in an aluminum electrolytic capacitor, leakage current increases with time. Thus, the aluminum electrolytic capacitor using an electrode foil produced by the above conventional process has a short lifetime.
Furthermore, a small-sized aluminum electrolytic capacitor cannot be obtained by the use of such a conventional electrode foil.